PURPOSE: To prevent explosion and firing of a combustible with certainty by controlling the feed rate of a nitrogen-enriched gas feed means according to the result of detection of the density of nitrogen or oxygen in a gaseous phase portion in contact with the combustible.

CONSTITUTION: A nitrogen-enriched gas storage bomb 11 in which a nitrogen- enriched gas for injection for the purpose of explosion proofing is stored is communicated with the gaseous phase portion 7a of a space above fuel 2 in a fuel tank 1 via a solenoid valve 9. Compressed air (engine exhaust gas) is allowed to pass through a filter 4 and a heater 6 used for preventing dewing of water in air at low temperatures and is introduced into a film separation type nitrogen separating device 5 and oxygen, carbon dioxide and nitrogen are separated from one another there and a nitrogen-enriched gas 7 is introduced into the gaseous phase portion 7a of the fuel tank 1 by piping 3. The nitrogen density is detected by a nitrogen density sensor 8 being exposed to the nitrogen- enriched gas 7 and the solenoid valve 9 is controlled by a control computer 10 according to whether the density is large or small and when nitrogen is insufficient the nitrogen-enriched gas in the bomb 11 is supplied.